Ruthenium-catalyzed epoxidation of unfunctionalized olefins with tert -butyl hydroperoxide

Tetrakdron, Vol. 52, No. 40, pp. 12971-12978, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights resewed 004042W96 $15.00 + 0.00

PII: SOO40-4020(96)00778-S

Ruthenium-Catalyzed Epoxidation of Unfunctionalized Olefins with &,&Butyl Hydroperoxide.

Gert A. Barf, Dennis van den Hoek and Roger A. Sheldon*. Department

of Organic Chemistry and Catalysis, Dclh University of Technology, Julianalaan 136, 2628 BL Dclft, the Netherlands.

Abstruct: A number of unfunctionalized aromatic and aliphatic olefins was tested in ruthenium-catalyzed epoxidation with tert-butyl hydroperoxide as the external oxidant. The effect of reaction without ligands and with two different type of ligands was investigated. Olefins containing terminal double bonds afforded low epoxide selectivities, whereas secondary and tertiary olefins afforded up to 80 % selectivity for the epoxide. A mechanism for the ruthenium-catalyzed epoxidation with tert-butyl hydroperoxide is proposed. Copyright 8 1996 Elsevier Science Ltd

Metal-catalyzed

epoxidations

organic synthesis. There systems

for the

of unfunctionalized

is also a considerable

enantioselective

olefins

interest

metal-catalyzed

constitute

in the development

epoxidation

of prochiral

an important

tool in

of effective catalytic olefins’.

The

major

advances in this field have been achieved with chiral porphyrins2 and chiral manganese(II1)

Schiff

base complexes, the latter developed by Jacobsen”t4 and Katsuki’. Ruthenium ability

to form

is a transition compounds

Ruthenium-catalyzed iodosylbenzene’,

epoxidations mentioned

in eleven

have

different

epoxidations’ pyridine

hydroperoxide13,

metal that is extremely versatile for oxidation reactions, based on its

N-oxides’“,

aldehyde/0214 also

have

been

and

oxidation been

states,

described

hypochlorite”ka”,

even

dioxygen’5v’“.

reported’7>‘x>‘” but

ranging

are

with

hydrogen

from sodium

yet

periodate7>8,

peroxide12,

Ruthenium-catalyzed not

RuT2 to Ru+~.

competitive

teti-butyl

enantioselective with

the

above

methods with porphyrins or Schiff base complexes.

In the absence of ligands, RuO,, in a stoichiometric

amount or as catalyst in combination

with

an oxidant, leads to oxidative cleavage of double bonds with aromatic olefins to afford aldehydes or ketones and to allylic oxidation with aliphatic reason that the small amounts reactions,

of epoxide, that were sometimes

could probably be enhanced

oxidizing power of Ru04 periodate

(NaI04)

olefins. Balavoine

by employing

This indeed

or sodium hypochlorite

an electron

and coworkers7 were the first to observed

as byproducts

donating

ligand to moderate

proved to be the case: reaction

in these the

of olefins with sodium

(NaOCI), in the presence of catalytic amounts of RuCl, 12971

12972

G. A.

and bipyridyl

or substituted

phenanthrolines,

et al.

BARF

afforded

the corresponding

epoxide

as the major

product7. The reaction was stereospecific for both &- and trams-alkenes, consistent with a heterolytic mechanism. presence

Interestingly,

the epoxidation

of styrene with tert-butyl hydroperoxide

of catalytic amounts of Ru(PPh,),CI,

had been described

epoxidation

with TBHP, affording 25 % styrene oxide. Since TBHP is much more interesting point of view, we have investigated

the possibilities

is mentioned

than NaI04, from

of using TBHP

in ruthenium-

catalyzed epoxidation

reactions.

catalyzed epoxidations

of unfunctionalized

the

(4S,5S)-( +)-4-hydroxymethyl)-5-phenyl-2-(2-pyridinyl)-4,5-dihydro[2,l-d]oxazole

chiral

ligand

in the

more than 10 years earlier by

Turner and Lyonst4. This is the only example where ruthenium-catalyzed

an industrial

(TBHP)

Here, we report our results on the use of TBHP in rutheniumolefins without and with the ligand bipyridyl (bpy, 1) and

(pymox, 2)20 (Figure 1).

Figure I. The structure of hpy (U und pyrwx

(21.

Results and Discussion

Both H,O,

and TBHP

are susceptible

mechanism for the decomposition initiated by a one-electron

is shown

to facile ruthenium-catalyzed

below (Scheme 1). The formation

of a tBu0.

The

radical is

reduction of TBHP by ruthenium.

tBuOOH + Flu”+

-

tBuO_ + R&+1)+

tBu0.

-

tBuO$ +

+

tBuOOH

2 tBuO,m -

2 tBu0. + 0,

Scheme 1. Ruthenium-initiuted

tBuOH

-

tBuOOtBu + 0,

decomposition

of

TBHP.

When RuCl, is added to a 70 % TBHP solution in water, spontaneous observed and within seconds all the TBHP is decomposed of oxygen. The rate of decomposition water-free environment.

decomposition.

Therefore,

is substantially

gas evolution (02) is

to afford the theoretical

decreased

to suppress the decomposition

if the reaction

amount (40 ml)

is performed

in a

of TBHP, a 2.7 M TBHP solution

Epoxidation of unfunctionalized olefins

in 1,2-dichloroethane21

was used and the reactions

using Ru(dmso)&12 22 as the source of ruthenium. the rate and amount of decomposition, the reaction

mixture,

using different

reaction at O°C gave a substantially

were performed

under water-free

conditions,

Some experiments

were performed

to quantify

by measuring the amount of molecular oxygen evolved from ligands or complexes

and temperatures.

lower rate of decomposition

period, probably because initially there is some free ruthenium TBHP with in situ generated

They showed that

than reaction at 20°C. After a short in the solution, the decomposition

of

complexes ceased (Figure 2). When an olefin (trans-I.5methylstyrene)

was added the total decomposition Ru(dmso)Jl,/bpy

12973

drops dramatically

to 15 ml of 0,

and 10 ml for Ru(dmso)&/pymox.

(of 40 ml theoretical)

This suggests that the olefin participates

for in

the complex formation. A

number

of

was

tested

with

Ru(dmso)J12,

Ru(dmso)&l,/bpy

(2). The results are shown in Table 1. The blank reaction

Ru(dmso)&12/pymox ruthenium)

olefins

afforded only low conversions

in all cases (8 % maximum

(reaction

and

without

for trans-&methylstyrene).

contrast to the reaction with NaIO,, where reaction without ligand afforded benzaldehyde product7>‘, here reaction without ligand afforded substantial

(1)

In

as the only

amounts of epoxide e.g. 79 % conversion

and 42 % selectivity for truns-stilbene.

40 =

E

30

f o f

20

”

" '. ,Or_-_-_---_-_-_-_-_----

0

”

”

".

..

_

Ru(dmso)&/bpy Ru(dmso),CI,/bpy/olefin _

Ru(dmso),Cl2/pymox/olefin

i 0

10

20

30

40

60

20

Tlrn.(rnl")

Figure 2. Cruph of evolution of 0, (40 ml at complete decomposition) from reaction of TBHP with bpy, bpy/olefin und pymox/olefin at 2@C.

During the reaction, the dimethyl sulfoxide in the GC-MS chromatogram

of the crude product). When ligands were used, the epoxide selectivity

reached from 65 % to 76 % for truns-stilbene Ru(bpy),Cl, epoxidation

and 78 % for cir-stilbene.

was much slower than with the in situ generated was stereospecific,

indicating some contribution decomposition

(dmso) ligands are oxidized (dimethyl sulfone was found

while with cis-stilbene

of one-electron

The reaction with preformed

complex. With trans-stilbene

the

a cis/truns mixture of 70:30 was observed,

transfer. It should be noted that, because the amount of

of TBHP may vary, 2-4 equivalents

of TBHP were used, sufficient for the completion

12974

G. A. BARF et al.

of each reaction. For the epoxidation

of trans-stilbene

was observed, again suggesting some contribution

using pymox

(2) as the ligand, an e.e. of 5 %

from one-electron

transfer since the epoxidation

with NaIO, (which proceeds only via a two-electron transfer) afforded 13 % e.e.=. Olefins with a terminal

double bond were not effective substrates

with TBHP. Styrene, a-methylstyrene

in epoxidation

reactions

and also I-octene gave poor results. In the case of styrene, this

is a result of instability of styrene oxide, which reacted further to give the chlorohydrin. Although the necessary

HCl could be derived

chlorohydrin CH,Cl,

from the catalyst, this could not account

formed. The only other possible explanation

presumably

methylstyrene

via initial

hydrogen

abstraction

for the amount

of

is that HCI is formed from the solvent, by a tert-butoxy

radical.

Both

rran.&

and indene afforded good selectivities in the presence of bpy and pymox. With indene

the reaction rate is also strongly enhanced

when the ligand is applied (from 24 to 5 hours reaction

time). Some aliphatic olefins were also tested. Again, I-octene with a terminal gave any conversion.

The other olefins, cyclooctene

and cis-2-heptene

respectively with pymox as ligand. However, no enantioselectivity

double bond hardly

afforded 61 % and 64 %,

was observed in these cases.

Mechanism of ruthenium-catalyzed epoxidation

tert-Butyl hydroperoxide

(TBHP) can act as either a one- or a two-electron

surprisingly, the oxidation of olefins with TBHP catalyzed by Ru(dmso)&

experiments

Oxidation of cyclobutanol cyclobutanone

were

carried

out

in the presence

products

cyclobutanol

products, mainly butyraldehyde

The latter are indicative of a homolytic mechanism.

oxidation

using

as a mechanistic

probe24.

with TBHP, in the presence of Ru(dmso)4C12, led to the formation of both

and ring-opened

was performed

in the absence of added

amounts of the epoxide (9 - 42 %, Table 1).

ligands afforded substantial Separate

oxidant. Quite

Similarly, when the oxidation of Ru(dmso)4C12

of the radical inhibitor

were observed,

consistent

and butyric acid (identified by gc-ms).

lonol (2,6-di-tee-butyl-4-methylphenol)

with a homolytic

pathway. Moreover,

observed with TBHP (5 % vs. 13 % with Na104x) in the Ru/pymox-catalyzed stilbene suggests a substantial

contribution

from one-electron

no

the low e.e.

epoxidation

of fruns-

transfer.

We propose the mechanism outlined in Scheme 2 to account for the observed results. Initial reaction and/or

of a ruthenium(I1) dioxoruthenium(V1)

radical via one-electron a terr-butylperoxy (di)oxoruthenium

complex with TBHP can involve the formation

complexes via a heterolytic mechanism or the formation

transfer. In the latter case the tet-butoxy

radical.

of oxoruthenium(IV)

Subsequent

epoxide

complex or a teti-butylperoxy

formation

of a tert-butoxy

radical reacts with TBHP to afford

results

from

radical with the olefin substrate.

reaction

of either

a

Epoxidation

Table 1. Epoxidations o/ wqiotcdonalized OldiIl

olefms

wilh

of unfunctionalized

12975

olefins

TBHP a1 @C’.

Catalyst

Conversion

Sclcclivit~

TBHP

Time

(o/u)

(%)

(ea.)

(h)

4

12

2

72

Ru(dmso)& Ru(dmso)&lJbpy Ru(bpy),Cl, Ru(dmso)4C12/pymox

79h SO 54 ‘99

42 76

2 3

24 5

61

2

24

65

3

30

Ru(dmsoLClJovmox

3 99

73= 7SC

2 4

68 30

rl 31 ‘99

4

66

Ru(dmso)4Clz Ru(dmso)4C12/pymox

17d n.d.d

4 4

48 30

Ru(dmso),& Ru(dmsoKIJovmox

0 41 98

9 5

4 4 4

66 48 72

8

13

3

20

rrans-B-methylstyrene

Ru(dmso)& Ru(dmso)4C12/bpy RufdmsoLClJovmox

97 9x W

27 57 56

3 3 3

2 2 3

24

Ru(dmso),C$ Ru(dmso)&l,/bpy Ru(dmsoLCIJovmox

0 9x 97 96

2

indene

26 77 So

2 2 2

24 5 6

5 99

n.d. 61

4

67

Ru(dmso)&l,/nvmox

4

72

4 4

72 72

4 4 4

43 48 40

rmns-stilbene

cis-stilbene

styrene

a-methylstyrene

cyclooctene

l-octene

tract Ru(dmso)4Clz/pymox

cis-Zheptene

1GlCC

1 15 95

Ru(dmso)& Ru(dmso),Cl,/ovmox

n.d. 22 64

(a) Selectivity of epoxide W. aldchydc/allylic oxidation products. (b) reaction at 6’C. (c) cis/mrrrs-ratio 70~30. (d) Styrene oxide proved to be unstable under the reaction conditions. GCMS showed that the chlorohydrin of styrene oxide was the main product.

In the presence accelerated

of bpy or pymox the reaction is both faster and more selective, Le. ligand-

catalysis is observed25. The higher selectivities

ligands more reaction these reactions

occurs via the putative

is the observation

than the in situ generated is a prerequisite preformed

catalyzed epoxidations.

intermediates.

that the reaction with preformed

complex. It suggests that prior coordination

for effective epoxidation.

oxoruthenium

oxoruthenium

suggest that in the presence

A puzzling feature

of

Ru(bpy)2C12 was much slower of the olefin to the ruthenium

Moreover, it implies that stoichiometric

complexes are not a good measure

of these

epoxidations

of what is happening

with

in ruthenium-

G. A. BARFet al.

12976

‘B

L,,FW+ + l-BuO,H

P

-

L+f’+

heterolytic oxidation

+ 1-BuOH

(0) in competition with: $Rtf’+

+ t-BuO,H

-

Mu0

+ t-BuO,H

-

q..,p+lO~

+ t_BuO’

homolytic oxidation

t-BuO 2’ + t-BuOH

Scheme 2. Proposed mechanism for ruthenium-cutulyzed epoxidations with TBHP as tire primury oxidant.

In conclusion,

ruthenium-catalyzed

epoxidations

involving both homolytic and heterolytic

of olefins

with TBHP

pathways. The mechanistic

are complex

processes

details of the oxygen transfer

step are currently under further investigation.

Acknowledgements

This research was supported

by the Innovative

the Dutch Ministry of Economic supply of RuCiYnH20

Oriented

Affairs. Johnson-Matthey

research Programm-Catalysis

(IOP-C) of

is gratefully acknowledged

for the kind

via the loan-scheme.

Experimental

General procedure for epoxidation of olefins with TBHP. 24 mg (0.05 mmol) of Ru(dmso)&

(and

3 equivalents of ligand) were dissolved in 25 ml of CH,CI,

and stirred with a magnetic stirrer for 1 h at 600 rpm. Then, 0.54 g (3 mmol) of trum-stilbene added

and the reaction

ClCH,CH,CI

mixture

was cooled to 0°C. 2-4 eq. Of a 2.67 M TBHP

was added with a Dosimat at a rate, similar to the reaction

solution

was in

rate. The reaction was

monitored by GC and ‘H-NMR and compared to authentic samples of the products.

12977

Epoxidation of unfunctionalized olefins

Decomposition of TRHP. In a closed reaction vessel 24 mg of complex (and ligand) in 12 ml CH,CI, was stirred and 1 ml of 4.4 M TBHP solution in CICH,CH,CI

was added. The amount of oxygen evolving was measured by

leading it into a column of water.

UV-Vis experiments with Ru(bpy)&I,

and TBHP.

To a solution of 26.0 mg of Ru(bpy)$I,

in 100 ml of

CH,CI,

and this mixture was allowed to stir for 2 days. The oxidation

were added 2 equivalents

was monitored

complete oxidation fran.s-stilbene was added. No change of the UV-spectrum

of TBHP

by UV-Vis. After

was observed.

Notes and References 1. 2.

3. 4. 5.

6. 7. 8. 9.

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(Received in UK 29 April 1996; revised 28 August 1996; accepted 29 August 19%)